1. Field of the Invention
The present invention relates generally to a zero turning radius vehicle. Specifically, the present invention provides a zero turning radius vehicle with outboard steerable front wheels and independently-driven rear wheels.
2. Background
Whether a lawn mower is utilized commercially or for personal use, efficient operation is essential. In commercial settings, time is money and contractors desire to finish a yard as quickly as possible so they may complete more jobs in a day. Similarly, a homeowner wants to finish his yard as soon as possible so that he may move on to more enjoyable activities. Zero-turning-radius (“ZTR”) vehicles adapted to include a mower deck have become particularly popular in the lawn care industry because their tight cornering capability obviates the constant need to shift gears from forward to reverse or to drive in a wide loop in order to turn around, as is necessitated by the large turning radius of a non-ZTR vehicle.
Zero turning radius vehicles known in the art have the capability to make turns having a center of rotation at the midpoint between the independent drive wheels. This is accomplished by driving the independent drive wheels at different speeds, and even in different directions, with respect to each other. ZTR vehicles are particularly useful in the lawn care industry. ZTR vehicles require the operator to facilitate steering of the vehicle by maneuvering separate levers responsible for controlling the independent drives of each wheel rather than providing a steering wheel. However, many users find the dual lever system extremely difficult to operate given that the vast majority of vehicles with which the average person is familiar utilizes a steering wheel as the steering input. The dual-lever systems typically involve a very lengthy “learning curve” to enable the user to become accustomed to its operation.
In commonly known ZTR vehicles, the drive wheels, or locomotion wheels are the rear wheels. These rear ground-engaging, independent drive wheels also provide the means of steering the vehicle. As a result, it is not necessary for the front wheels to be steerable. Therefore, caster wheels are typically used for the front ground-engaging wheels. Caster wheels spin 360 degrees about a vertical axis of rotation, but are not steered. Rather, they simply respond to the movement and direction of the vehicle as dictated by the drive wheels.
The problem with these non-steerable front wheels, however, is that “crabbing” is a major problem. For example, when a ZTR vehicle using non-steerable caster wheels as front ground-engaging wheels is driven laterally across the side of a hill, the gravitational force of the lawn mower tends to pull the vehicle down the side of the hill. The natural tendency of the caster wheels is to turn down the hill even if the operator does not wish to turn in that direction. To counteract this phenomenon, users of common ZTR vehicles must continually provide intermittent steering inputs with the levers to maintain the ZTR vehicle on the hill. The result is a series of zigzag motions (or “crabbing”) as the vehicle traverses the hill or an angling of the vehicle as it moves across the hillside.
In systems that purport to “steer” the front wheels, the front wheels are simply linked at all times to the drive wheels so that some amount of “bias” is always present. “Bias” in this context refers to a situation where the rear drive wheels are being operated at differing speeds and/or direction from each other. Moreover, the steering wheel in such systems is used at all times as an accelerator. Every steering input provides bias to the drive wheels. There is no mechanism allowing the steering wheel to not be used as a biasing accelerator. This would appear to cause major problems for the user. First, it could be difficult to control the vehicle since any steering input is automatically translated into a bias condition. Second, constantly biasing the drive wheels could create the need for constant steering inputs, always correcting and counter-correcting for each previous steering input. Linear lines of travel could be difficult, if not impossible, to achieve. Moreover, constantly biasing the rear wheels could tear up the turf.
The need exists for a ZTR vehicle that can actually steer the front wheels of the vehicle separate from the drive wheels in order to provide more efficient and effective operation and reduce or eliminate the crabbing phenomenon. The need also exists for a ZTR vehicle that is more user-friendly so that a consumer need not spend an inordinate amount of time learning how to operate the vehicle. Also, the need exists for a ZTR vehicle that provides a multistage steering input to steerable front wheels and to drive wheels in a predetermined relationship to eliminate the constant bias in the drive wheels, thus providing a much more natural vehicle similar to other steered-wheel vehicles in use.